1. Field of the Invention
The invention relates to the field of forming gas-tight seals in missile bodies and in particular to a methodology of simultaneously forming a structural and gas-tight bond between the fuselage of the missile body and the forward projecting radome of the missile body.
2. Description of the Prior Art
In many applications, the interior of a missile body is filled with gas under pressure, such as nitrogen or other inert gas mixtures. It is necessary that the missile body remain gas-tight not only during handling and flight, but also during long periods of storage. Loss of the gas pressure and contamination from outside gases and moisture will either entirely disable the missile, or subject it to a serious probability of degradation of performance.
Therefore, the prior art had devised various methods for assembling and sealing the various sections of the missile body together, particularly the ceramic radome to the metallic fuselage. These prior art methods required extensive processing time and skill. Typically, five different materials were required to establish the bonding to the missile body, namely a film adhesive, a paste adhesive, a polysulfide sealant, a thickener or thickening agent and a preformed packing. These five elements, all included within a single joint or bond, were utilized to fabricate a seal which required fourteen assembly steps. One of the steps required curing the bond at three different temperatures for four or five hours. In addition, two different assembly fixtures were required, a first vacuum fixture used to manually inject the polysulfide into a gap between the radome and fuselage, and a second alignment fixture used to maintain proper alignment of the assembly in the elevated temperature cure. Furthermore, a final finishing step required a fourteen-day cure at room temperature prior to the performance of any leak testing to determine whether or not the bond had been adequately formed. In addition to the inherent complexity of the methodology, it also had to be very carefully performed to avoid defects in the seal, which required the attention of a skilled and experienced fabricator.
Turn to FIG. 1, which is a cross-sectional view of the radome-to-fuselage seal and joint area depicted as formed by the prior methodology. A radome 10 is structurally bonded through a gas-tight seal, described below, to a fuselage 12. Radome 10 is fitted within an inner flange portion 14 of fuselage 12. A Cab-O-Sil and polysulfide sealant 16 forms the bedding adhesive for base 18 of radome 10 which is seated against an O-ring 20. O-ring 20 has an inner diameter sized to accommodate the outer diameter of flange portion 14. O-ring 20 is disposed against butt flange surface 22 of fuselage 10 and the Cab-O-Sil, a trademark of Cabot Co. and polysulfide sealant 16 fills the space radially outside of O-ring 20 and between radome 10 and fuselage 12. The space between flange portion 14 and radome 10 is similarly filled with HT-424 film and paste adhesive manufactured by American Cyanimid.
The steps of prior art fabrication are as follows. Radome 12 is cleaned by vapor degreasing with trichloroethane. Thereafter the radome is removed from the vapor bath and cooled to room temperature. The fuselage bonding surface, the surface portion of fuselage 12 which will be provided as a bonding surface to radome 10, as depicted in FIG. 1, is wiped with acetone or methylethyl ketone. After wiping it is flushed with either acetone or methylethyl ketone and any excess solvent is removed. Fuselage 12 is allowed to air-dry at room temperature. An etching solution is prepared by adding chromium trioxide, sodium fluoride and sulfuric acid in an aqueous solution. The bonding surface of fuselage 12 is then etched in this solution. The etched fuselage is then rinsed with tap water and subsequently rinsed with deionized water. The fuselage is dried in an oven at 150 degrees F. for approximately ten minutes. The fuselage is then placed within a fabrication jig with a lower positioning ring strapped around the fuselage approximately three inches above its base. A seven-inch preformed packing O-ring 20 is then rinsed with acetone and placed around fuselage 12 on butt flange surface 22. A strip of frozen HT-424 film is applied to the bonding surface of the fuselage with the bottom of the adhesive strip just contacting the top of O-ring 20. The end strips of the adhesive are butted together without gaps or overlap.
The fuselage thus prepared is placed with its jig in a freezer at -40 degrees F. or colder. HT-424 paste, also manufactured by American Cyanimid, is a two-part paste which is now mixed. A thin film of HT-424 paste is applied to the interior bonding surface of radome 10 starting approximately 1/8 inch above the base of the radome and is spread evenly with a flat applicator. A bonding fixture is blown dry and cleaned while the cold fuselage is removed from the freezer. Radome 10 is then carefully mated to fuselage 12 so as to avoid squeeze-out of any adhesive in the bond gap. An upper positioning ring is placed upon radome 10 and aligned with respect to the positioning ring of fuselage 12. The radome and fuselage are then carefully placed within a bonding fixture with the positioning rings that are disposed on the radome and fuselage aligned with index marks in the bonding fixture. The rings are then secured to the bonding fixture by appropriate means and/or installed with alignment pins and then the fixture's threaded rods are tightened until the gap between the radome and fuselage is between 0.055 to 0.070 inch. The precise centering of the radome is confirmed in the bonding jig by means of an alignment pin which when properly aligned easily slips into the radome tip.
The bonding fixture together with the radome and fuselage is then placed into an oven and preheated to 220 degrees F. for a minimum of sixty minutes. Thereafter the temperature is increased to approximately 275 degrees F. and allowed to cure for a minimum of 120 minutes. The temperature is increased still further to 340 degrees F. for a minimum of 120 minutes. After the three-step oven treatment, the assembly is removed from the oven and allowed to cool to room temperature. After achieving room temperature the assembly is removed from the bonding fixture.
A polysulfide sealant and Cab-O-Sil thickener, manufactured for example by American Cyanimid, are then mixed according to the desired proportions and degassed. An enclosure is then placed on or around the fuselage which is connected to a vacuum pump. Bonding material is then injected into the gap using a syringe with an appropriate tip at maximum pressure while the interior of the fuselage and radome are evacuated. The gap is filled to a level just above a flush condition. Any cavities which may form are refilled. The assembly is then left for fourteen days at room temperature to cure.
After cure any excess bonding material is cut with an knife so that the bonding material is flush with the radome/fuselage surface.
The radome can now be tested to determine whether or not the seal is leak-tight. If the seal has been successfully made the radome/fuselage assembly is then prepared for appropriate painting, labelling, coatings and assembly with the remaining portions of the missile.
Therefore, what is needed is a methodology for bonding gas-tight seals in a missile body wherein the number of material elements in the seal may be reduced, wherein the number of process steps may be reduced, wherein the number of fixtures which are required during the fabrication may be reduced, and wherein the curing time and fabrication time may be reduced.